1. Field of the Invention
The present invention relates generally to machine tools, and more particularly to a tapping apparatus for cutting screw threads inside cavities of workpieces by power feeding a tap.
2. Description of the Prior Art
In conventional machine tools such as a machining center, a tap is attached to one end of a spindle thereof and the machine head supporting the spindle is moved toward a workpiece in synchronization with rotation of the spindle.
A rotary encoder is mounted on the spindle of the machining center in order that the head of the same is moved in synchronization with rotation of the spindle. The rotary encoder performs the arithmetic-logical operation to obtain the rotational speed of the spindle, thereby controlling movement of the machine head. Accordingly, it is mechanically difficult to have the movement of the machine head agree with a pitch of the tap. Therefore, the prior art has provided an improvement that a chuck is mounted on the spindle with a coil spring interposed therebetween so that the chuck is slightly moved in the axial direction. In case that a workpiece has a small thickness, however, the spring force acting on the chuck reduces accuracy of the screw thread. Further, in case that the machining center is not provided with a rotary encoder, it cannot be used to cut screw threads by means of a tap.